Entity-Relationship Model. Database Modeling (Part 1) Entity. Entity-Relationship Model EMPLOYEE

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1 Entity-Relationship Model Database Modeling (Part 1) A conceptual data model, which is a representation of the structure of a database that is independent of the software that will be used to implement the database DB212 Peter Lo DB212 Peter Lo Entity-Relationship Model Entity Name Address EMPLOYEE Employee No. Skill name DB212 Peter Lo An Entity can be a person, place, object, event or concept in the user environment about which the organization wishes to maintain data. Examples: Person: EMPLOYEE, STUDENT, PATIENT Place: STATE, REGION, COUNTRY Object: MACHINE, BUILDING Event: SALE. REGISTRATION Concept: ACCOUNT, COURSE Entity DB212 Peter Lo

2 Entity Type and Entity Instance. There is a difference between Entity Type and Entity Instance. Entity Type is a collection of entities that share common properties or characteristics Entity Instance is a single occurrence of an entity type Examples Entity type:employee Attributes: EMPONO NAME ADDRESS YEARHIRED Instances: Roy Lim Mary Wong Hong Kong Kowloon DB212 Peter Lo DB212 Peter Lo Attribute Attribute is a property or characteristic of an entity that is of interest to the organization. Both entity and relationships may have attributes. Example: STUDENT: EMPLOYEE: STUDENT NO.,NAME, ADDRESS EMPLOYEE NO., NAME, SKILL Candidate Key Candidate key is an attribute (or combination of attributes) that uniquely identifies each instance of an entity type Candidate Key Attribute DB212 Peter Lo DB212 Peter Lo

3 Primary Key Primary Key is a candidate key that has been selected as the identifier for an entity type. Primary Key Multivalued Attribute Attribute that can have more than one value for each entity instance. For example An EMPLOYEE may possess a number of skills. So SKILL is a multivalued attribute. DB212 Peter Lo DB212 Peter Lo Relationship Relationships are what that holds together the various entities.. Degree of Relationship The degree of a relationship is the number of entity types that participate in that relationship Unary relationship (recursive relationship) Binary relationship Ternary relationship Relationship DB212 Peter Lo DB212 Peter Lo

4 Unary Relationship Unary Relationship (Recursive Relationship) Unary Relationship also called Recursive Relationship It is a relationship between the instances of one entity type PERSON Is married to 1 1 One-to-one DB212 Peter Lo DB212 Peter Lo Unary Relationship (Recursive Relationship) Unary Relationship (Recursive Relationship) ITEM Has components N M Many-to-many DB212 Peter Lo DB212 Peter Lo

5 Binary Relationship It is a relationship between instances of two entity types. Ternary Relationship It is a simultaneous relationship among instances of three entity types. Ternary relationship is a simultaneous relationship among instances of three entity types DB212 Peter Lo DB212 Peter Lo Ternary Relationship Existence Dependency WAREHOUSE An instance of one entity cannot exist without the existence of an instance of some other entity. VENDOR Ships DB212 Peter Lo 2004 WAREHOUSE 19 DB212 Peter Lo

6 Weak Entity Example of Weak Entity A Weak Entity is an entity type that has an existence dependency. A relationship in which the primary key of the parent entity is used as part of the primary key of the dependent entity. Weak entities usually do not have a natural identifier because the primary key of the parent entity is often used as part of the primary key of the dependent child entity. Data integrity is enforced as weak entity cannot exist unless the parent entity exists. Student No. STUDENT Student Name Has Student No. PARENT Parent Name DB212 Peter Lo DB212 Peter Lo Generalization Generalization is the concept that some things (entities) are subtypes of other, more general things. To express generalization relationships, object are arranged into a hierarchy. Example: Accountant, Programmer Analyst are subtypes of the more general type called STAFF. Categorization Categorization is when an entity comes in various subtypes. Example: There are different subtypes of employee which are hourly employee, salaried employee and part-time employee. DB212 Peter Lo DB212 Peter Lo

7 Supertypes and Subtypes Supertypes A generic entity type that is subdivided into subtypes. Subtypes A subset of a supertype that shares common attributes or relationships distinct from other subsets. ISA Relationship The relationship between each subtype and supertype is called an ISA relationship. The subtypes are mutually exclusive and that one (i.e. required for each instance of the supertype). HOURLY EMPLOYEE Address ISA Employee no. EMPLOYEE ISA SALARIED EMPLOYEE Name Date Hired ISA Daily rate CONSULTANT Hourly rate Annual salary Stock option Contact no. DB212 Peter Lo DB212 Peter Lo 2004 Employee Employee Employee 26 no. no. no. Logical Database Design There is a process of transforming the conceptual data model into a logical database model. There are four types of logical database models in use today: Object-oriented Hierarchical Network Relational. Object-Oriented Data Model Most future database management systems will be based on objects, or will incorporate objectoriented functionality. This enable users to create generic, all purpose components that can be reused in multiple applications. DB212 Peter Lo DB212 Peter Lo

8 Object Attribute Objects are abstraction of the real world entities that exhibit states and behaviors. The state of objects are expressed as values of the Attributes of the object. The behavior of an object is expressed by a set of Methods that operate on its attributes. These are the properties of objects that are of interest to the organization. Attributes describe the characteristics of an object. Objects can have a specific attribute called a state. The state of an object describes the object's current status. DB212 Peter Lo DB212 Peter Lo Methods Encapsulation Methods define the behavior of an object. Can only process data within the object class in which they are defined (Encapsulation). Can receive requests from methods in another object class. A number of method categories: Occur Methods instance add, instance change, instance delete. Calculate Methods perform calculations on the data values encapsulated in the same object class. Monitor Methods produce signals when predetermined limits are exceeded in a system. Encapsulation is the property that the attributes and the methods of an object are hidden from the outside world and do not have to be known to access its data values or use its methods. DB212 Peter Lo DB212 Peter Lo

9 Object Classes and Instance Identifying and Describing Objects A logical grouping of objects that have the same attributes and behavior. An object instance is one occurrence of an object class. When we use the term object by itself, we are referring to an object class. Object name Attributes Methods VEHICLE Number Year Model AddVehicle DB212 Peter Lo Top-down Approach Bottom-up Approach DB212 Peter Lo Top-down Approach Bottom-up Approach This begins with a high level description of the environment and proceeds from the general to specific. In the object-oriented data model, methods are activated by sending message from a sending object to a receiving object. For example studying written material and talking with users are activities to locate nouns in written material. This provides Information about Potential Objects. Bottom-up approach begins with system detail, example reports, video forms and other detail documents and displays. The analyst identifies the Candidate Objects and their Properties. DB212 Peter Lo DB212 Peter Lo

10 Top-down vs. Bottom-up Approach In reality, the top-down and bottom-up approaches should be used to identify and describe candidate objects. Inheritance Inheritance is an important principle of the objectoriented mode. Inheritance means that all properties of an object class become the properties of its subclasses. DB212 Peter Lo DB212 Peter Lo Inheritance Example of Inheritance EMPLOYEE Inheritance is the property that, when entity types or object classes are arranged in a hierarchy, each entity type of object class assumes the attributes and methods of its ancestors. All properties of an object class become the properties of its subclasses. Only attributes that are unique to a subtype are associated with that subtype. The attributes of EMPLOYEE apply to all three subclasses. The method CalculateAge applies to all employees. Hourly rate HOURLY Employee no. Name Address Date hired Birthdate CalculateAge SALARIED Annual salary Stock option CONSULTANT Contact no. Date hired DB212 Peter Lo DB212 Peter Lo CalculateMthlyWage CalculateStockBenefit AllocateToContract

11 Advantage of OO Approach Reusability Objects can be defined for a variety of functions and then reused in numerous applications. Complex data types An object-oriented database can store and manage complex data such as documents, graphics, images, voice message and video sequences. Distributed databases Object-oriented databases can support distribution of data across a network more easily than other data models. DB212 Peter Lo Appendix Entity-Relationship Diagrams DB212 Peter Lo Entity-Relationship Diagrams An Entity-Relationship Diagram (ERD) is a graphical model of the information system that depicts the relationships among system entities. Three Main Types of Relationships One-to-One relationship (1:1) One-to-Many relationship (1:M) Many-to-Many relationship (M:N) DB212 Peter Lo DB212 Peter Lo

12 One-to-one relationship (1:1) It exists when exactly one of the second entity occurs for each instance of the first entity. One-to-Many Relationship (1:M) It exists when one occurrence of the first entity can be related to many occurrences of the second entity, but each occurrence of the second entity can be associated with only one occurrence of the first entity. DB212 Peter Lo DB212 Peter Lo Many-to-Many Relationship (M:N) It exists when one instance of the first entity can be related to many instances of the second entity, and one instance of the second entity can be related to many instances of the first entity. Cardinality Cardinality describes how instances of one entity relate to instances of another entity. DB212 Peter Lo DB212 Peter Lo

13 Cardinality Symbols DB212 Peter Lo DB212 Peter Lo Creating an Entity Relationship Diagram (ERD) 1. Identify the entities. 2. Determine all significant events, transactions, or activities that occur between two or more entities. 3. Analyze the nature of the interaction. 4. Draw the Entity Relationship Diagram. DB212 Peter Lo